JPH052266B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Application Field] The present invention relates to a gas sensor for detecting carbon monoxide gas generated from various combustion appliances, blast furnaces, etc., or generated at various chemical factories. [Prior art] Gas sensors that have been developed to detect carbon monoxide (hereinafter referred to as CO) include (1) wet type constant-potential electrolysis type sensors, and (2) semiconductor type sensors (mainly sintered bodies of tin oxide). ), (3) those using a catalytic combustion method, and (4) those using an oxygen ion conductive solid electrolyte. These CO detection principles are different and are as follows. That is, method (1) catches the current generated when CO is oxidized to CO ₂ at the reaction electrode and O ₂ is reduced to H ₂ O at the counter electrode. Method (2) measures the change in the conductivity of a semiconductor when CO is adsorbed, and method (3) measures the temperature change when CO is combusted by a catalyst as a change in the resistance value of the platinum electrode. be.
In addition, method (4) uses CO on one side of the zirconia chip.
A non-combustible porous layer is arranged on one side, and a H ₂ and CO combustible porous layer is arranged on the other side, and the conduction of oxygen ions from one side to the other side during CO combustion is extracted as an electrical signal. . However, each of these previously known sensors has drawbacks. Type (1) requires maintenance such as updating the electrolyte because the concentration of the electrolyte changes (due to water evaporation, etc.) during long-term use, and type (2) has less sensitivity to hydrogen than sensitivity to CO. Since the operating temperature is kept low (for example, below 100℃) in order to obtain high CO selectivity, moisture and oil in the air cannot be automatically evaporated or decomposed, and therefore regular high-temperature purge operations are required. shall be. The method (3) cannot achieve high sensitivity because it uses an indirect detection method, and the method (4) has a complicated structure and is large, so it consumes a lot of power to maintain the operating temperature.
Not suitable for mass production. [Problems to be Solved by the Invention] Of the types (1) to (4) above, the present invention is directed to type (2), that is, a semiconductor type CO using a sintered body of tin oxide.
This is an attempt to improve gas sensors, specifically at temperatures of about 300°C, which is much higher than the 100°C mentioned above.
The present invention aims to provide a CO gas sensor that can be operated at temperatures above 350°C, and in some cases around 450°C, thereby eliminating the above-mentioned drawbacks and exhibiting excellent CO selectivity. [Means for solving the problem] As a result of various studies to solve this technical problem, the present inventor found that by adding an alkali metal oxide to a tin oxide semiconductor, CO selectivity during high-temperature operation can be significantly improved. We learned that it can be improved. The present invention was completed based on this knowledge. That is, according to the present invention, an oxide of at least one alkali metal selected from sodium, potassium, rubidium, and cesium is added to the tin oxide semiconductor in an amount of about
Provided is a carbon monoxide gas sensor characterized in that carbon monoxide is added in an amount of 0.5 mol% or more. More preferably, when the oxide is sodium oxide, it is added in an amount of about 1.5 to 5.0 mol% based on tin oxide, and when the oxide is potassium oxide, rubidium oxide, or cesium oxide, it is added in an amount of about 0.5 to 5.0 mol% based on tin oxide.
The composition is said to be 10 mol% added. Lithium behaves slightly differently from other homologous elements,
Francium is excluded from the preferred embodiment because the effect of improving CO selectivity is relatively small, and francium is excluded because it is a rare element that exists only in the decay series of radioactive elements and is not practical. If the amount of alkali metal oxide added to tin oxide (SnO ₂ ) is less than 0.5 mol %, the above effects will be insufficient, so it is not preferable.
In the case of potassium oxide, rubidium oxide, or cesium oxide, if it is used at a relatively low temperature (e.g. around 300°C), it will have an adverse effect on the tin oxide itself and its fine sintered structure due to its strong alkalinity if it exceeds 10 mol%. This is still not desirable because it affects the environment. In the above, the method for "adding" may be appropriately selected from among the following impregnation methods, mixing methods, etc. The impregnation method here refers to immersing a sintered body of tin oxide in a solution of the above-mentioned soluble alkali metal compound.
This refers to a method in which the impregnated compound is decomposed into an oxide, and the mixing method refers to a method in which tin and alkali metal hydroxides are thoroughly mixed and then sintered. [Effect] The details of the mechanism by which alkali metal oxides improve the CO selectivity of tin oxide semiconductors are unknown, but H ₂ as a coexisting gas reacts with, for example, Na ₂ O and K ₂ O to form metal hydroxides. It is presumed that since tin oxide tends to form, it is at least captured by these metal oxides and therefore becomes less likely to affect the conductivity of tin oxide. [Effects of the Invention] As mentioned above, the CO gas sensor of this invention has the above-mentioned advantages.
Since it belongs to type (2), it has advantages such as simple and compact structure, low power consumption, suitable for mass production, and maintenance-free.In addition, the unique effect of the present invention is that it is suitable for high-temperature operation. It exhibits excellent CO selectivity and eliminates the need for periodic high-temperature purge operations that were conventionally required during low-temperature operation. [Example] A. Production of gas sensor First, the production method of the gas sensor of the present invention will be explained using the above-mentioned impregnation method. Commercially available tin tetrachloride (SnCl ₄ ) is used as a starting material, and an aqueous solution of a certain concentration is prepared. The tin hydroxide precipitate obtained by dropping ammonia water into this aqueous solution is dried and then fired in an electric furnace to obtain semiconductor tin oxide. This is crushed into a fine powder, kneaded with water to form a paste, and this paste is applied to a noble metal coil 1 as a detection electrode of a gas sensor as shown in Fig. 1 to form a sphere 2 with a diameter of approximately 0.5 mm. (which will become the semiconductor section 6 described later). After drying, a predetermined current is passed through the precious metal coil 1 to heat it to a predetermined temperature, e.g.
Sintering tin oxide powder at 800℃. Although the external shape remains unchanged as shown in Fig. 1 due to sintering, microscopically it has become porous. On the other hand, since an aqueous solution of water-soluble salts of alkali metals, such as nitrates and acetates, is prepared, a porous tin oxide sintered body is immersed in this solution, dried, and then energized again. and thermally decompose the nitrate or acetate. As a result, the nitrate and acetate groups decompose and volatilize, and the alkali metal becomes an oxide.
The fine particles 3 are supported on the surface of the tin oxide particles 4 as shown in FIG.
becomes. Note that FIG. 2 is an enlarged view of the area surrounded by circle A in FIG. 1. B Measurement Results of CO/H ₂ Sensitivity Ratio The gas sensor 5 produced as described above is usually used by being incorporated into a wheast stone bridge circuit as shown in FIG. The number 7 is a resistor connected in series thereto as a load resistance for the gas sensor, and the numbers 8 and 9 are resistors connected in series with each other to determine the reference potential of this circuit. Gas sensor 5 and resistor 7 are connected to power supply 1 for other resistors 8 and 9.
parallel with respect to 0, each intermediate point 11, 12
The unbalanced potential difference between them is detected by the voltmeter 13 as an output voltage (mV). The value obtained by subtracting the output voltage (Va) in clean air from the output voltage Vg in the presence of gas is displayed as sensitivity (ΔV) below. ΔV=Vg−Va and ΔV(CO) is carbon monoxide detection sensitivity,
If ΔV(H ₂ ) is defined as hydrogen gas detection sensitivity, then
The CO/H ₂ sensitivity ratio is expressed as ΔV(CO)/ΔV(H ₂ ). In the following tests, CO gas is set at 100 ppm, and hydrogen gas is set at 1000 ppm, which is 10 times that value, so even if the sensitivity ratio above is 1, the CO gas sensitivity is H ₂
This means it is 10 times more sensitive. The level of CO selectivity that is recognized as practically sufficient is when the sensitivity ratio is about 0.5 or more, that is, when the CO sensitivity is about 5 times or more than the H ₂ sensitivity. Table 1 below shows the operating temperature of several samples with varying amounts of potassium oxide (K ₂ O), rubidium oxide (Rb ₂ O), and cesium oxide (Cs ₂ O).
The sensitivity ratio measured at 390°C is shown.

【table】

[Table] Table 2 shows the results when the amount added was 4 mol% and the operation was performed at 390°C. Next, by setting the CO gas concentration to two levels, 100 ppm and 200 ppm, and changing the voltage applied to the Wheatstone bridge (that is, the voltage of the power supply 10).
The operating temperature of the gas sensor with _Rb2O4mol % addition is 370
Figure 4 shows a graph when changing the temperature in the range from ℃ to 430℃. However, the vertical axis shows ΔV instead of the sensitivity ratio.
(CO) or ΔV(H ₂ ) is taken in mV. As is clear from Tables 1 to 2 and Figure 4 above,
Addition of alkali metal oxide to tin oxide
The CO selectivity is good at 0.5 mol% or more, and even better at 1.5 to 5 mol%, and there is considerable quantification with respect to changes in CO concentration, and the optimum operating temperature is
It is in the range of 370 to 430℃, and shows the best CO sensitivity especially at 390 to 410℃, and in terms of CO selectivity,
It was confirmed that 370 to 390°C is more desirable.

[Brief explanation of the drawing]

The figures show an embodiment of the present invention; Fig. 1 is a front view of a gas sensor, Fig. 2 is an enlarged cross-sectional view of a portion thereof, Fig. 3 shows a circuit incorporating the gas sensor, and Fig. 4 shows performance. This is the graph shown. 1...Precious metal coil, 2, 6...Semiconductor part, 3...
...fine particles of alkali metal oxide, 4... particles of tin oxide, 5... gas sensor.

Claims (1)

[Claims] 1. A tin oxide semiconductor containing sodium, potassium,
Approximately 0.5 mol of at least one alkali metal oxide selected from rubidium and cesium
% or more of carbon monoxide gas sensor. 2. The carbon monoxide gas sensor according to claim 1, wherein the oxide is sodium oxide, and this is added in an amount of about 1.5 to 5.0 mol% relative to tin oxide. 3. The monoxide according to claim 1, wherein the oxide is an oxide of an alkali metal selected from potassium, rubidium, and cesium, and is added in an amount of about 0.5 to 10 mol% based on tin oxide. Carbon gas sensor.